1. Field of the Invention
The present invention relates to fiber amplified spontaneous emission (ASE) light sources, and more particularly, to superfluorescent fiber sources that have a stable mean wavelength with respect to changes in pump polarization.
2. Description of the Related Art
Fiber ASE light sources are well known in the art. ASE sources have been advantageously used to provide wideband (e.g., on the order of 10 to 30 nanometers), spatially coherent light for multiple applications. For example, ASE sources have been used to provide laser light as an input to a fiberoptic gyroscope. For a description of an exemplary superfluorescent fiber source, see an article entitled xe2x80x9cAmplification of Spontaneous Emission in Erbium-Doped Single-Mode Fibersxe2x80x9d by Emmanuel Desurvire and J. R. Simpson, published by IEEE, in xe2x80x9cJournal of Lightwave Technology,xe2x80x9d Vol. 7, No. May 5, 1989.
An ASE light source typically comprises a length of single-mode fiber, with a portion of its cross-section (typically the core) doped with an ionic, trivalent rare-earth element. For example, neodymium (Nd3+) and erbium (Er3+) are rare-earth elements that may be used to dope the core of a single-mode fiber so that it acts as a laser medium.
The fiber receives a pump input signal at one end. The pump signal is typically a laser signal having a relatively narrow spectrum centered around a wavelength xcexp. The ions within the fiber core absorb the input laser radiation at wavelength xcexp so that electrons in the ground state of these ions are excited to a higher energy state of the ions. When a sufficient pump power is input into the end of the fiber, a population inversion is created (i.e., more electrons within the ions are in the excited state than are in the lower laser state), and a significant amount of fluorescence is generated along the length of the fiber. As is well known, the fluorescence (i.e., the emission of photons at a different wavelength xcexs) is due to the spontaneous return of electrons from the excited state to the lower laser state so that a photon at a wavelength xcexs is emitted during the transition from the excited state to the ground state. These photons are amplified by the gain as they travel down the fiber, leading to amplified spontaneous emission (ASE). The light which is emitted at the wavelength xcexs from the fiber is highly directional light, as in conventional laser light. However, one main characteristic of this emission which makes it different from that of a traditional laser (i.e., one which incorporates an optical resonator) is that the spectral content of the light emitted from the superfluorescent fiber source is generally very broad (typically several tens of nanometers). This principle is well known in laser physics, and has been studied experimentally and theoretically in silica-based fibers doped with erbium, neodymium, or other rare earths, for several years.
Light emitted from ASE fiber sources has multiple applications. For example, in one application, the output of the ASE source is fed into a fiberoptic gyroscope. For reasons that are well understood by those skilled in the art, the fiberoptic gyroscope should be operated with a broadband source which has a highly stable mean wavelength. Of the several types of broadband sources known to exist, superfluorescent fiber sources, in particular, made with erbium-doped fiber, have been thus far the only optical sources which meet the stringent requirements for inertial navigation grade fiberoptic gyroscopes. The broad bandwidth of light produced by erbium-doped fiber sources, together with the low pump power requirements and excellent mean wavelength stability of erbium-doped fiber sources, are the primary reasons for use of such sources with fiberoptic gyroscopes.
In an erbium-doped fiber, the emission of a superfluorescent fiber source is bi-directional. That is, the light which is emitted by the return of electrons to the ground state in the erbium ions is typically emitted out of both ends of the fiber. As described in U.S. Pat. No. 5,185,749, to Kalman, et al., for erbium-doped fibers of sufficient length, the light propagated in the backward direction (i.e., in the direction opposite that in which the pump signal propagates) has a very high efficiency. Thus, it is advantageous to implement erbium-doped sources so that the light emitted from the ASE erbium-doped source is emitted from the pump input end of the fiber (i.e., in the backward propagation direction).
An ASE source is generally implemented in one of two configurations. In a first configuration, called a single-pass ASE source, the superfluorescent source output power is emitted in two directions, one of which is not used. In the second configuration, called a double-pass ASE source, a reflector is placed at one end of the doped fiber to reflect the superfluorescent source signal so that the superfluorescent signal is sent a second time through the fiber. Since the fiber exhibits gain at the superfluorescent signal wavelengths, the ASE signal is further amplified. One advantage of the double-pass configuration is that it produces a stronger signal. A double-pass ASE source configuration also produces output only at one port (i.e., in one direction). A disadvantage of such a configuration is that the feedback optical signal from the gyroscope must be kept very low in order to prevent lasing (e.g., with use of an optical isolator located between the source and the gyroscope).
For fiberoptic gyroscope applications, one critical measure of source performance is the stability of the source mean wavelength (for example, see U.S. Pat. No. 5,355,216 to Kim, et al.). As is well known in the art, stability of the source mean wavelength leads directly to the stability of the gyroscope scale factor. Precise knowledge of the scale factor is critical for an accurate measurement of the rotation rate of the gyroscope. Presently, superfluorescent fiber sources exist which have a mean wavelength stability with respect to pump power, pump wavelength, temperature, and level of optical feedback down to a few parts per million each, assuming reasonable stabilization of system parameters such as pump wavelength, pump power, temperature and optical feedback from the gyroscope. However, an overall stability of better than one part per million in mean wavelength is desirable for some applications, in particular, high-grade fiberoptic gyroscopes.
Polarization effects have recently been shown to play a role in the instability of the mean wavelength of superfluorescent fiber sources (SFS). The polarization dependence of the mean wavelength of an SFS output has been predicted through numerical modeling by J. L. Wagener, et al. [see J. L. Wagener, xe2x80x9cErbium doped fiber sources and amplifiers for optical sensors,xe2x80x9d Ph.D. thesis, Applied Physics Department, Stanford University (March 1996); J. L. Wagener, M. J. F. Digonnet, and H. J. Shaw, xe2x80x9cA High-Stability Fiber Amplifier Source for the Fiber Optic Gyroscope,xe2x80x9d J. Lightwave Technol. Vol. 15, 1689-1694 (September 1997); and J. L. Wagener, D. G. Falquier, M. J. F. Digonnet, and H. J. Shaw, xe2x80x9cA Mueller Matrix Formalism for Modeling Polarization Effects in Erbium-Doped Fiber,xe2x80x9d J. Lightwave Technol. Vol. 16, 200-206 (February 1998), which are hereby incorporated by reference herein]. These studies have shown that the mean wavelength of the SFS depends slightly on pump polarization. The reason for this can be explained in physical terms as follows. The ions of erbium (or another dopant, such as Nd or another rare earth) in the fiber host experience an intrinsic anisotropy of absorption and emission with respect to polarization. For example, some erbium ions more strongly absorb a given polarization than others, and correspondingly, these erbium ions have a preferred polarization associated with their enission. This effect gives rise to polarization-dependent gain when the erbium-doped fiber is pumped in the usual manner, i.e., by a highly polarized source such as a laser diode. This in turn can result in orthogonal polarization components of the output ASE signal having different mean wavelengths.
A first embodiment of the invention is a superfluorescent source that includes an optical pump source that generates optical radiation that is substantially unpolarized and an optically active solid state medium (e.g., a solid state laser medium) that is pumped by the substantially unpolarized optical radiation. The medium has characteristics selected to generate superfluorescence having a full width at half maximum (FWHM) of at least 2 nm and a mean wavelength that is stable to within 50 ppm against (i.e., even in the presence of) polarization fluctuations in the superfluorescent source. In one preferred embodiment, the superfluorescence has a mean wavelength that is stable to within 3 ppm in the presence of polarization fluctuations in the superfluorescent source. In a preferred embodiment, the mean wavelength is stable to within 50 ppm in the presence of birefringence changes in the superfluorescent source. In one preferred embodiment, the mean wavelength is stable to within 50 ppm in the presence of polarization changes of the optical radiation from the optical pump source.
Another embodiment of the invention is a superfluorescent source that includes an optical pump source that generates optical radiation that is substantially unpolarized and an optically active solid state medium (e.g., laser medium) that is pumped by the substantially unpolarized optical radiation. The medium has characteristics selected to generate superfluorescence having a full width at half maximum (FWHM) of at least 2 nm and a mean wavelength that is stable to within 50 ppm even in the presence of polarization changes in the source that range over the Poincaire sphere.
Yet another embodiment is a superfluorescent source that includes an optical pump source that generates optical radiation that is substantially unpolarized. The optical pump source includes a plurality of pumps that generate respective optical outputs, a polarization mixer that receives the respective optical outputs from the plurality of pumps and generates optical output (in which the respective optical outputs from the plurality of pumps have polarizations selected such that optical output from the mixer is substantially unpolarized), and a depolarizer that receives the optical output from the polarization mixer. The embodiment further includes an optically active solid state medium (e.g., laser medium) that is pumped by the output from the depolarizer, in which the medium has characteristics selected to generate superfluorescence having a full width at half maximum (FWHM) of at least 2 nm and a mean wavelength that is stable in the presence of polarization fluctuations in the superfluorescent source. In a preferred embodiment, the superfluorescent source has a mean wavelength that is stable to within 500 ppm in the presence of polarization fluctuations in the superfluorescent source. In a more preferred embodiment, the mean wavelength is stable to within 100 ppm in the presence of polarization fluctuations in the superfluorescent source. In a still more preferred embodiment, the mean wavelength is stable to within 50 ppm in the presence of polarization fluctuations in the superfluorescent source. In a most preferred embodiment, the mean wavelength is stable to within 3 ppm in the presence of polarization fluctuations in the superfluorescent source. In one preferred embodiment, the plurality of pumps includes two pumps having respective optical outputs whose polarizations are combined so that their polarizations are orthogonal to each other. In a preferred embodiment, the mean wavelength is stable to within 500 ppm in the presence of birefringence changes in the superfluorescent source. In one preferred embodiment, the mean wavelength is stable to within 500 ppm in the presence of polarization changes of the optical radiation from the optical pump source.
Yet another preferred embodiment is a method of generating superfluorescence, which includes providing a plurality of optical pumps having respective optical outputs with different polarizations, directing the respective optical outputs through a polarization mixer that produces optical output (in which the different polarizations are selected so that the optical output from the mixer is substantially unpolarized), depolarizing the output from the mixer, injecting the depolarized output into an optically active solid state medium (e.g., laser medium), and producing superfluorescence from the medium that has a mean wavelength that is stable in the presence of polarization fluctuations in the superfluorescent source. In a preferred embodiment, the wavelength is stable to within 500 ppm in the presence of polarization fluctuations in the superfluorescent source. In a preferred embodiment, the plurality of optical pumps includes two pumps having respective optical outputs combined so that their polarizations are orthogonal to each other.
Yet another embodiment is a method of generating superfluorescence, comprising providing an optically active medium (e.g., laser medium) having first and second ends, pumping the first end of the medium with optical output from a first optical pump (in which the output from the first optical pump has a first power and a first polarization), and pumping the second end of the medium with optical output from a second optical pump (in which the output from the second optical pump has a second power and a second polarization different from the first polarization). The method further includes producing optical output from the first end of the medium that comprises a first spectral component having a first mean wavelength and a polarization parallel to the first polarization, and a second spectral component having a second mean wavelength and a polarization orthogonal to the first polarization. The method also includes selecting the first pump power and the second pump power so as to substantially reduce the polarization dependent gain that would be present if the first power were equal to the second power, so that the difference between the mean wavelength of the first spectral component and the mean wavelength of the second spectral component is substantially reduced. In a preferred embodiment, the first polarization and the second polarization are orthogonal. In one preferred embodiment, the second power is selected to be less than the first power.
Another embodiment of the invention is a device that includes an optical pump that produces polarized optical output. The source further includes an optically active, solid state medium (e.g., laser medium) that receives the polarized optical output, in which the medium has birefringence axes that receive equal amounts of pump power to reduce polarization dependent gain effects within the medium. The medium produces optical output that has substantially the same mean wavelength for all polarization. The device further includes a fiber optic gyroscope that receives the optical output from the medium.
Yet another embodiment of the invention is a method of generating superfluorescent optical output that includes outputting a polarized optical signal from a pump source (in which the polarized optical signal has a polarization axis), inputting the polarized optical signal into an optically active, solid state medium (e.g., laser medium) that has birefringence axes, and orienting the birefringence axis of the solid state medium at about 45 degrees with respect to the polarization axis of the polarized optical output to reduce polarization dependent gain effects within the medium such that the solid state medium produces a superfluorescent optical output that has substantially the same mean wavelength for all polarizations.
Still another embodiment of the invention is a method of generating superfluorescent output from a superfluorescence source that includes providing an optical pump which generates optical output and directing the optical output into a polarization mixer which generates a first output signal and a second output signal (in which the two output signals having respective intensities and different polarizations). The first output signal is directed into a first end of a optically active solid state medium (e.g., laser medium), and the second output signal is directed into a second end of the optically active solid state medium. Optical gain is produced in the solid state medium that is substantially independent of polarization to generate optical output from one end of the solid state medium whose mean wavelength is stable even in the presence of polarization fluctuations in the superfluorescent source. In a preferred embodiment, the gain that is substantially independent of polarization is produced by selecting the intensities of the first and second output signals.